1. Field of the Invention
The present invention relates to an aluminum nitride powder to be used for the production of aluminum nitride sintered body.
2. Description of the Prior Art
The surfaces of the particles of an aluminum nitride powder have heretofore been composed of aluminum oxide boehmite (AlOOH) and/or aluminum hydroxide (Ai(OH).sub.3) formed by the reaction with the moisture in the air. In the case of the powder produced by the reductive nitriding method in which decarbonizing is effected in an atmosphere containing oxygen gas in order to remove excess carbon at the time of production, the surfaces of the particles have been composed of an aluminum oxide (Al.sub.2 O.sub.3) formed in the decarbonizing step. A number of cracks has been caused on the surface layer thereof due to the difference in thermal expansion coefficient between aluminum nitride and aluminum oxide, boehmite and/or aluminum hydroxide, or due to milling of the aluminum nitride powder.
Accordingly, with regard to the conventional aluminum nitride powder, there has been observed a phenomenon that reaction takes place between the oxygen in the surface layer, which is composed of aluminum oxide, boehmite and/or aluminum hydroxide, and the aluminum nitride inside the particles due to the heating accompanying sintering, causing an increase in the amount of oxygen in solid solution inside aluminum nitride crystals and an a decrease in the amount of oxygen in the surface layer at the same time. As a result, the conventional aluminum nitride has brought about the problems that the resultant aluminum nitride sintered body is lowered in thermal conductivity and changed in color tone, and pores are formed in the grain boundary region by the decrease in a liquid phase amount and nonuniform distribution thereof which accompany the decrease in the amount of oxygen in the surface layer.
An aluminum nitride powder reduced in oxygen content to attain a high thermal conductivity has problems that the thickness and distribution of the surface layer are made nonuniform and, therefore, when the powder is allowed to stand at room temperature in the atmosphere, moisture and oxygen in the atmosphere penetrate through the microcracks at the time of heating or with the elapse of time, thus causing an increase in the amount of oxygen and finally an increase in the amount of oxygen in solid solution inside the grains of aluminum nitride (AlN).